Thermally activated delayed fluorescence (TADF) compounds with a flexible donor–acceptor structure suffer from conformational disorder in solid-state, which deteriorates their emission properties as well as OLED performance.

Thermally activated delayed fluorescence (TADF) materials, combining high fluorescence quantum efficiency and short delayed emission lifetime, are highly desirable for application in organic light-emitting diodes (OLEDs) with negligible external quantum efficiency (EQE) roll-off. Here, we present the pathway for shortening the TADF lifetime of highly emissive 4,6-bis[4-(10-phenoxazinyl)phenyl]pyrimidine derivatives. Tiny manipulation of the molecular structure with methyl groups was applied to tune the singlet–triplet energy-level scheme and the corresponding coupling strengths, enabling the boost of the reverse intersystem crossing (rISC) rate (from 0.7 to 6.5) × 10 6 s –1 and shorten the TADF lifetime down to only 800 ns in toluene solutions. An almost identical TADF lifetime of roughly 860 ns was attained also in solid films for the compound with the most rapid TADF decay in toluene despite the presence of inevitable conformational disorder. Concomitantly, the boost of fluorescence quantum efficiency to near unity was achieved in solid films due to the weakened nonradiative decay. Exceptional EQE peak values of 26.3–29.1% together with adjustable emission wavelength in the range of 502–536 nm were achieved in TADF OLEDs. Reduction of EQE roll-off was demonstrated by lowering the TADF lifetime.

TADF compounds with flexible molecular core show large conformational disorder and emission wavelength instability in solid films.

Construction of rigid TADF compounds allows us to minimize the conformational disorder and obtain single-exponential DF in solid hosts with an exceptional RISC rate of nearly 6 × 106 s−1 and high emission yield.

Time-resolved emission spectra of thermally activated delayed fluorescence (TADF) compounds in solid hosts demonstrate significant temporal shifts. To explain the shifts, two possible mechanisms were suggested, namely, slow solid-state solvation and conformational disorder. Here we employ solid hosts with controllable polarity for analysis of the temporal dynamics of TADF. We show that temporal fluorescence shifts are independent of the dielectric constant of the solid film; however, these shifts evidently depend on the structural parameters of both the host and the TADF dopant. A ≤50% smaller emission peak shift was observed in more rigid polymer host polystyrene than in poly(methyl methacrylate). The obtained results imply that both the host and the dopant should be as rigid as possible to minimize fluorescence instability.